The invention relates interferometry apparatus and methods, more particularly, to interferometry apparatus and methods for measuring the profile of a surface on an industrial part.
Interferometric techniques are commonly used to measure the profile of a surface of an object. To do so, an interferometer combines a measurement wavefront reflected from the surface of interest with a reference wavefront reflected from a reference surface to produce an interferogram. Fringes in the interferogram indicate spatial variations between the surface of interest and the reference surface. For example, a single-wavelength visible interferometer using phase-shifting techniques can measure surface variations on the order of Angstroms. Such accuracy is important in applications such as characterizing of glass substrates used in lithographic photomasks.
Another type of interferometer is a white-light scanning interferometer, which uses a white-light source and scans the relative path length of the reference and measurement legs of the interferometer. Because the white-light source has a limited coherence length, interference fringes in the interferogram are only present where the optical path difference (OPD) between the measurement and reference paths for corresponding points on the surface of interest and the reference flat is less than the coherence length. Thus, the scanning interferometer can resolve a step, or an otherwise large and/or discontinuous, variation in the surface of interest by scanning the relative path length of the reference and measurement legs, recording multiple interferograms, and determining where in each interferogram interference fringes are present. See, e.g., N. Balasubramanian in U.S. Pat. No. 4,340,306 for additional information regarding scanning interferometers. Furthermore, phase-shifting techniques can be combined with scanning interferometry to interpolate the phase of individual fringes and thereby more accurately determine surface variations.
Complications with interferometric techniques arise, however, when the surface of interest is rough on the scale of the illumination wavelength. The rough surface can scatter the measurement wavefront and corrupt the phase of whatever interference fringes are present in the interferogram. Nonetheless, recent studies have shown that white-light scanning interferometry can be used to measure the profile of a rough surface. In particular, when the surface of interest is sufficiently rough relative to the illumination wavelength, the interferogram includes speckle. Although the speckle corrupts the phase of any interference fringes, they nonetheless exhibit intensity oscillations as a function of OPD when the OPD is within the coherence length of the white-light source. Thus, white-light scanning interferometry can determine the surface profile of a rough surface to within about the coherence length. See, e.g., T. Dresel, G. Hausler, and H. Venzke, in Applied Optics 31:919-925 (1992).